1. Field of the Invention
This invention relates to a thermal recording system for recording information on a recording medium by a thermal printhead which includes a plurality of heat-producing elements arranged in the form of an array, and in particular, to such a thermal recording system using a unregulated supply as a power source.
2. Description of the Prior Art
A thermal recording system is well known in the art and its typical structure is illustrated in FIG. 7. As shown, the thermal recording system includes a power supply 10 connected to an a.c. commercial line and a thermal print head 20 which includes a plurality of electrically resistive elements R which are arranged in the form of a single row. As will be described later in detail, the plurality of resistors R are selectively activated in accordance with image information supplied thereto to produce heat to "burn" points on a recording medium which is moved in contact with the array of resistors R in the direction perpendicular to the longitudinal direction of the row, as well known in the art.
As shown in FIG. 7, the thermal print head 20 is also provided with a shift register SR having the capacity to store bits of information corresponding to the number of resistors R. The shift register SR receives image information DI through its serial input terminal serially. Also provided is a latch L which has the same capacity as that of the shift register SR, whereby each bit of the latch L is connected to the corresponding one of the bits of the shift register SR. Thus, when the latch L receives a load signal LD, the image information then stored in the shift register SR is transferred into the latch L in parallel. Also provided are a plurality of AND gates G each of which has its one input connected to the corresponding one bit of the latch L and having the other input connected to receive a strobe signal SB. Each AND gate G has its output connected to the base of a corresponding switching transistor Tr which has its collector connected to one end of the corresponding resistor R. Each of the transistors Tr has its emitter connected to ground, and the other end of each of the resistors R is connected to a common line 1", to which a desired driving voltage is applied from the power supply 10, as will be described more in detail later.
In such a line-type thermal print head, there are provided a large number of resistors R, typically, in the order of 1,000 to 2,000, so that, for example, in the case of a thermal print head having the resolution of 8 dots per mm and the recording width of 200 mm, its power supply is required to supply several hundreds W of power if it is desired to activate all of the resistors R at the same time for recording all black across the width of the line array. In order to mitigate the power requirement, the resistors R may be activated as divided and staggered in time; in this case, however, it is often required that a power supply be of the capacity in the order of 100 W. If the power supply 10 is assumed to have the capacity of 10 V, there will be a flow of current in the order of 10 A between terminals VHD.sub.1 and GND.sub.1, and, thus, if either of lines 1 and 1' does not have a sufficient current capacity, there will be created an appreciable voltage drop. The same arguments hold true for the common line 1" inside of the thermal print head 20. Such a voltage drop will cause the recording density to deteriorate so that locally high and low density areas will be created. On the other hand, to provide a power supply having the capacity of supplying 100 W or more is not advantageous from an economical viewpoint.
As disclosed in the Japanese Patent Laid-open Pub. No. 52-19542, in the case of a battery-powered thermal printer, there has been proposed a method for maintaining the recording density constant by controlling the activation time period based on monitoring the applied voltage. In this voltage-controlled thermal printer, the activation time period is controlled in accordance with the power supply voltage to be applied to the electrical resistors to keep the level of energy applied to the resistors constant and thereby provide stable recording density. In the case of the device disclosed in the above-referenced Japanese application, it is battery-powered and of the serial printing type, wherein the number of dots to be printed at the same time is extremely small. However, in the case of a thermal print head for use in a line printer, which has attracted much attention lately and is coming into wide use, it is required to operate at high speed with large current, so that any voltage drop in the interconnection between the power supply and the thermal print head can not be neglected, thereby making it difficult to maintain stable recording density.